In U.S. Pat. No. 3,653,385 to Charles Burton there is described a method and apparatus for applying heat to body tumors and particularly to brain tumors. The apparatus comprises a needle inserted into the tumor and an r.f. coil located external to the body. The needle consists of electrically conductive ferromagnetic material having a Curie temperature in the range of 150.degree. F. to 200.degree. F.
The frequency of the r.f. source is in the 300 to 400 KHz range and the coil is positioned such that the flux produced by the coil is coupled to the ferromagnetic material. The flux induces a current in the ferromagnetic material and skin effects become involved. The material is heated by the circulating current thus produced.
Needed improvements in the performance of the Burton device include increasing the heating power and making the device bio-compatible, i.e., capable of being inserted in living tissue for extended periods of time.
Recent disclosures by Matsuki and Murakami (H. Matsuki and K. Murakami, "High Quality Soft Heating Method Utilizing Temperature Dependence of Permeability and Core Loss of Low Curie Temperature Ferrite," IEEE Transactions on Magnetics, Vol. Mag-21, No. 5, September 1985) and by Matsuki et al (H. Matsuki et al, "An Optimum Design of a soft Heating System for Local Hyperthermia," IEEE Transactions on Magnetics, Vol. Mag-23, No. 5, September 1987) describe a needle comprising a ferrite rod and a non-magnetic metal ring into which the ferrite is inserted. An RF coil also located external to the body, produces flux in the ferrite which in turn causes currents to flow in the metal ring. Heat is produced in the ring by these induced currents.
There are numerous problems with the Matsuki et al devices which must be overcome before a practical implant or seed can be realized. The more important deficiencies are set forth. Ferrites are very brittle materials and in order to be used in the environment contemplated by the present invention must be relatively thick; too thick to be useful in tumor treatment of the brain and some other regions of the body. Also due to the thickness required, ferrites provide a poor length to width ratio. The materials used must be bio-compatible but those used in Matsuki et al are not. The total heat transfer is poor because only a part of the surface of the ferrite is enclosed in the ring. The ring is heated and in the region in which it is located transfers heat directly to the tumor. Along those regions of the seed where the ring is not located, heat must be transferred from the ring back into and spread through the ferrite which is a poor heat conductor. Thus efficiency suffers. Also ferrites are not ferromagnetic materials but are ferrimagnetic materials which have lower saturation magnetizations than the ferromagnetics.
Application of ferrites to hyperthermia is possible only if the needle is contained within a mechanically rigid, strong, and non-electrically conductive enclosure. Non-electrically conductive bio-compatible materials have low thermal conductivity, a factor which can seriously degrade the temperature regulating properties of the heater.
A German application, no. P12-84-528.2 published Dec. 5, 1968 for Device For Elimination of Biological Tissue Through Induction-Heating Objects With Temperatures-Stabilizing Properties by Dr. Friedrich Forster discloses the use of implant of ferromagnetic material enclosed within a non-magnetic conductor that is compatible with human or animal tissue. Such metal may be gold, silver or other bio-compatible conductive material. The implant is excited by an externally generated magnetic field developed by a current carrying coil or coils. One problem with the Forster disclosure is that it recites a number of parameters that must be considered but gives no idea of what they should be or their interrelationship. Also he uses material of low maximum permeability and between these two problems cannot achieve the desired results without exposing the patient to intense or prolonged magnetic fields.
The present invention makes possible for the first time a practical needle implant device incorporating the features of biological compatibility and effective internal generation and transmission of heat to the surrounding tissue.